Add slash events for slash points

We should emit an event when we add slash points. But we don't want to emit event every time we make an observation/etc. That would cause too much chain bloat. Maybe we don't include observations slash points, but only other slash point events, like not observing, not signing, etc.

We should include in the event..

1. Node address being slashed
2. The amount of slash points
3. the reason